The background relating to fuel dispensing nozzles is fully set forth in my U.S. Pat. No. 4,056,131, and my U.S. Pat. No. 4,057,086, both hereby incorporated by reference.
To briefly summarize, in filling a vehicle's fuel tank, hydrocarbon vapors are displaced, and it is desirable to return these vapors to the underground fuel reservoir rather than discharge them into the atmosphere. For this purpose, both the fuel dispensing nozzles of the aforementioned patents have vapor return lines through which vapors from the tank are collected and carried back to the reservoir. If the vapor return line is blocked or if the vehicle tank is subjected to a high vacuum condition, the tank will be subjected respectively to a large positive pressure or a large negative pressure. It is extremely desirable that the nozzle shut off automatically under both these conditions. It is also desirable that the vapor return line be completely sealed when the nozzle is not pumping gasoline.